This invention relates to a modulation and demodulation system suitable for use with a modem (modulator and demodulator apparatus) of the first polling type, and more particularly to a modulation and demodulation system wherein, upon transmission of data, training data of a particular pattern are modulated and transmitted prior to transmission of the data, and such training data are demodulated by demodulation means and initialization equalization processing of a reception section of the modulator and demodulator apparatus is performed using the demodulation training data.
FIG. 13 shows a general construction of an on-line system. Referring to FIG. 13, in the on-line system shown, a plurality of modems 203 are connected to a host computer 201 by way of a communication control apparatus (CCP) 202, and each of the modems 203 is connected by way of an analog circuit 204 to another modem 203' installed at another location. A terminal 205 is connected to each of the modems 203'.
The on-line system further includes a network supervisory apparatus 206, for which a secondary channel is used.
By the way, a state signal of a modem can be transmitted, from each of the host side modems 203 shown in FIG. 13, as it is to the network supervisory apparatus 206, but from each of the terminal side modems 203', a state signal thereof is transmitted to the associated host side modem 203 so that it is transmitted by way of the modem 203 to the network supervisory apparatus 206.
Since a state signal of a modem must necessarily be transmitted without having any influence on main data, each of the modems 203 and 203' divides, for example, a voice band of 0.3 kHz to 3.4 kHz by frequency division to provide a secondary channel for secondary data in addition to a main channel for main data as seen in FIG. 14.
It is to be noted that phase shift keying (PSK), quadrature amplitude modulation (QAM) or some other modulation is used for a main signal while frequency shift keying (FSK) is used for a secondary signal.
Meanwhile, a modem is constructed such that, upon transmission of data, training data of a particular pattern are modulated and transmitted prior to transmission of the data, and such training data are demodulated by demodulation means and initialization processing of a reception section of the modulator and demodulator apparatus is performed using the demodulation training data.
In particular, a modem includes, in its reception section, in addition to a demodulation section, a roll-off filter, an automatic gain control section (AGC), an automatic equalization section (AEQ), a carrier phase correction section (CAPC), a timing extraction section, a carrier detection section and so forth. Those components of the modem must necessarily be initialized upon starting of transmission of data. An optimum signal necessary for such initialization is, for example, a tone signal for an automatic gain control section, an impulse signal for an automatic equalization section, a tone signal or an impulse signal for a carrier phase correction section, a .pi./.pi. signal (two signals having phases different by 180.degree. from each other) for a timing extraction section and a tone signal for a carrier detection section.
Therefore, training data of a particular pattern are transmitted so that optimum signals (optimum patterns) may be supplied to the various components of the modem.
An exemplary one of training patterns which satisfy the requirement is shown in FIG. 16(a). Referring to FIG. 16(a), the training pattern shown includes a first repeat pattern portion 301 having a signal arrangement wherein signals A and B whose phases of signal points are different by 90.degree. from each other are arranged alternately, and a second repeat pattern portion 302 following the first repeat pattern portion 301 and having a signal arrangement wherein signals B and C whose phases of signal points are different by 180.degree. from those of the signals A and B and different by 90.degree. from each other are arranged alternately.
When signal points of the signals A to D on a phase plane are represented by the same reference characters as those used to represent the signals, if it is assumed that, as shown in FIG. 8(a), the point P1 represents the pattern A, the point P2 represents the pattern B, the point P3 represents the pattern C and the point P4 represents the pattern D, then the first repeat pattern portion 301 includes an alternate arrangement of the pattern A and the pattern C whose phases of signal points are different by 180.degree. from each other.
It is to be noted that the arrangement of signal points may alternatively be such an arrangement of the points Q1 to Q4 as shown in FIG. 8(b) or of the points R1 to R4 as shown in FIG. 8(c).
In order to reproduce an impulse from such a training pattern as shown in FIG. 16(a), such a circuit as shown in FIG. 15 is used. Referring to FIG. 15, when such a training pattern as shown in FIG. 16(a) is inputted to the point a in FIG. 15, a delay tap T of a first sum circuit 401 (refer to the point b in FIG. 15) provides such an output as shown in FIG. 16(b), and consequently, an adder of the first sum circuit 401 (refer to the point c in FIG. 15) provides such an output as shown in FIG. 16(c). Then, when the output of the first sum circuit 401 (refer to the point c in FIG. 15) is inputted to a second sum circuit 402, a delay tap T of the second sum circuit 402 (refer to the point d in FIG. 15) provides such an output as shown in FIG. 16(d), and consequently, an adder of the second sum circuit 402 (refer to the point e in FIG. 15) provides such an output as shown in FIG. 16(e). Then, such a reproduction impulse as shown in FIG. 16(f) is obtained from the signal of FIG. 16(e).
It is to be noted that, since a tone component and a .pi./.pi. component are included in the training signal of the BABA . . . pattern, a tone signal and a .pi./.pi. signal can be reproduced by processing the training signal by required calculation processing.
By the way, it is required for modems in recent years to establish multiple point connection in addition to a rise of the communication rate to reduce the cost of the circuit. To this lend, it is an effective technique to divide a frequency band of a main channel into a plurality of bands to transmit a plurality of data by way of the same circuit. However, where the technique is employed, since the roll-off ratio of the main channel is reduced very low, the number of taps of a roll-off filter must be increased, which results in increase of the filter transient. Consequently, the leading-in time of the timing filter is increased so long that a training pattern which has been employed may not possibly allow convergency of the timing phase within the training time. The same subject arises also when it is tried to assure a high modulation rate within a limited available frequency band.